1. Field of the Invention
The invention relates to a projector, and in particular relates to a projector with electromagnetic interference reduction.
2. Description of the Related Art
In general, a projector has a light machine normally comprising two holes formed on the back allowing calibration. No elements can be disposed on the back of the light machine. In addition, to reduce costs, a main circuit board, rather than a high-cost flexible print circuit board, is utilized to drive digital micromirror devices, and must generally be disposed above the light machine.
In such a structure, electromagnetic interference (EMI) may occur. The circuit board is covered by a sealed or semi-sealed shell to avoid EMI. Because the circuit board has many wires extending therefrom, a semi-sealed shell is preferred. A copper post connects the semi-sealed shell to a housing of the projector, whereby noise is directed to the housing to prevent EMI.
The structure described has more grounding points than conventional projectors and thus provides a good grounding function. As mentioned, since no element can be disposed on the back of the light machine, the main circuit board and the semi-sealed shell are only supported by the copper post, which stands alone without other contacting elements, thus acting as an antenna and generating additional EMI problems.
When the main circuit board emits electromagnetic waves, every λ/4 position (λ being the wavelength of the electromagnetic wave) a grounding point is required to avoid EMI. The correlation between λ, light speed c and frequency f is λ=c/f. For example, when f is 1 GHz, λ is 30 cm, and λ/4 is 7.5 cm. The copper post, at 10 cm exceeding λ/4 (7.5 cm), acts as an antenna a emitting electromagnetic waves, becoming another EMI source.